Multiple use wireless communications system and method

ABSTRACT

System and method for operating a wireless telephone system particularly a wireless Private Branch Exchange (PBX), as both a voice system and a pager system. The system includes a protocol for communicating both voice conversations and display data to a remote unit. Remote units, which may be pagers, personal communications units, mobile telephones, and the like, communicate with the wired portion of the system using the same protocols and signalling techniques.

RELATED APPLICATIONS

This application is one of a group of applications filed on even dateherewith, specifically: "A Wireless Communications System and MethodUsing a Reusable Control Channel", by Jackson et al., Ser. No.08/671,724; "A Multiple Orientation, Multiple Antenna Apparatus", byJackson, et al., Ser. No. 08/672,274; "A System For CommunicatingDigital Information Between A Base Unit and Plural Mobile Units", byJackson, et al., Ser. No. 08/671,896; "A Wireless Communications Systemand Method Having Dynamic Reallocation of Communications Frequencies",by Jackson, et al., Ser. No. 08/671,528; "A Method and Apparatus ForReducing Power Consumption In Wireless, Mobile Communicating Devices",by Hadar et al., Ser. No. 08/671,733; "A Method and Apparatus ForDetermining Symbol Timing In A Wireless Communications System", byHadar, et al., Ser. No. 08/672,269; "A Method and Apparatus For UsingDuality To Remotely Communicate", by Klein, et al., Ser. No. 08/671,727;"A Method and Apparatus For Configuring Dispersed Components In ACommunications System", by Jackson, et al., Ser. No. 08/671,670 and, "AMultiple Use Wireless Communications System and Method", by Jackson, etal., Ser. No. 08/671,615. Each of the forelisted applications are hereinincorporated by reference.

BACKGROUND OF THE INVENTION

The present application is directed in general to systems and methodsfor communicating in telephone systems and in particular to systems andmethods for using portable, wireless telephones integrated with aland-based wired telephone system.

The telephone industry has experienced tremendous growth in the last fewyears, driven in part by the availability of relatively inexpensivecellular telephone service. In such service, telephone subscribers useportable, battery-powered, hand-held telephone instruments tocommunicate via RF links with a network of base stations which interfacethe signals on the RF links into the land-based public telephonenetwork. In such systems, a cellular configuration is generally used inwhich a particular base station uses certain frequencies to communicatewith portable telephones within its "cell" and adjacent base stationsuse other frequencies to communicate with telephones within their cells.If base stations are sufficiently distant from each other, the samefrequencies may be used in both cells so long as neither the basestation nor the telephones in one cell generate signals of sufficientpower to carry into the other cell. In this way, a limited number offrequencies can be re-used in a non-conflicting pattern which providestelephone service throughout a particular geographic area. Of course, asa telephone leaves from one cell to another, the telephone may have toswitch frequencies (or "hop" to a new channel) during the middle of anon-going call to avoid broadcasting into a non-adjacent cell which isusing the same frequency for a different call.

The use of cellular telephone technology has permitted broad geographicregions to be provided with the availability of many portabletelephones. As the popularity of portable telephones has expanded,however, the frequencies (or "channels") have become filled. Within thelimited RF bandwidth available for all the competing uses thereof, thesolution to cellular overcrowding does not necessarily lie in merelyadding additional frequencies or channels within each cell.

It has been proposed in the prior art to utilize smaller cells and toreduce the power of the cellular equipment so that frequencies may bereused in greater proximity to one another. Such a solution is notentirely satisfactory, however, as with much reduced power levels, thesignal to noise ratios of the communications may approach levelsunacceptable in telephone communications. Along these lines, the priorart has proposed making an office building, a portion of an officebuilding or an office complex a cell site or a small cellular system.Given the crowding on the existing cellular systems, however, suchproposed systems are generally limited in the number of portabletelephones which they can accommodate.

In many office situations, the persons working in the offices frequentlyare away from their assigned duty locations for extended periods of timeand may even roam throughout the office as their business needs arise.Providing telephone service to such roaming persons in the prior artoften meant using call-forwarding schemes or the like to automaticallyroute the call from one location to another. Such systems usuallyrequired substantial user input to implement, a drawback in a typicaloffice environment. Thus, it is desirable to utilize portable orcellular technology for persons within an office. In a typical officebuilding, the addition of cellular telephones to the already-presentland-based system, such as a Private Business Exchange (or "PBX")provides some mobile communication but is insufficient for effectivecommunication. Generally, the portable telephones are tied to a baseunit which integrates the RF communications of the cellular system withthe public land-based telephone system and not directly with thebusiness person's own PBX. Effectively, such a solution in an officeenvironment yields two independent telephone systems not well tied intoeach other or coordinated in any way. Additionally, the externalstructure of the building housing the office may make impractical thetransmission of typical cellular RF signals to and from conventionalbase stations.

A solution to the desire to permit workers to roam within a particulargeographic space, such as an office building or a portion of an officebuilding, yet permit such workers to continue to be reachable by thetelephone system and to permit the workers to utilize the advantages ofthe telephone system is to integrate a wireless system with a PBX. Withsuch an integration, incoming calls to workers can be directed by thewireless system to the worker wherever that person may be within theoperating range of the system. At the same time, the roaming worker willbe able to readily place telephone calls both within and without theoffice as readily as if sitting at his own desk on the hard-wiredsystem. The roaming worker would also have the benefit of all of thefeatures of the PBX (such as a personalized repertory dialer, specialaccess to restricted circuits, etc.) no matter where the worker may belocated within the office.

Desirably, a wireless-PBX system would include a portable telephone unitwhich is easy to use, lightweight and has an extended battery life. Thelatter two desires are somewhat at odds with each other inasmuch asbattery life often has a direct relation to the size (and weight) of abattery. Accordingly, it is often important in the design of portableunits that they be as power-efficient as possible.

Within prior art cellular systems it is known for a particular basestation to have associated therewith one or more control channels andseveral or numerous voice channels. The control channels are used tocommunicate with the control portions of the portable units tocoordinate the use of voice frequencies and to perform an analog oftypical central office functions (e.g., dialing a number, ringing theportable unit, etc.). When all of the voice channels within a basestation are being utilized, there may no longer be a need for a controlchannel as there are no more control tasks to be performed until one ofthe voice channels again becomes available. In such systems, it isdesirable to not waste the non-functioning voice channel and anadditional voice channel can be obtained to enlarge system capabilitiesover conventional systems with no additional hardware costs and noadditional bandwidth being occupied by the system.

Accordingly, it is an object of the present invention to provide a novelsystem and method to integrate a wireless telephone system into aprivate branch exchange or similar office telephone system.

It is another object of the present invention to provide a novel systemand method to utilize every available channel of a wireless system forvoice (or data) messages.

It is yet another object of the present invention to provide a novelsystem and method for reducing power consumption within a portable unitwithout reducing capabilities of such a unit.

It is still another object of the present invention to provide a novelsystem and method which can be installed in an office environmentwithout extensive wiring and/or substantial programming.

It is a further object of the present invention to provide a novelsystem and method of providing an integrated wireless-PBX telephonecapability using housings which can be readily mounted on existing wallor ceiling surfaces of an office.

It is yet a further object of the present invention to provide a novelsystem and method of communicating in a wireless system in which basestations may readily be added to the system (or reset after a poweroutage) and brought on-line without extensive programming.

It is still a further object of the present invention to provide a novelsystem and method for accurately and efficiently communicating betweenplural wireless telephones and plural base stations.

These and many other objects and advantages of the present inventionwill be readily apparent to one skilled in the art to which theinvention pertains from a perusal of the claims, the appended drawings,and the following detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified functional block diagram illustrating anembodiment of the system of the present invention.

FIG. 2 is a timing diagram showing the relationship of frames and slotswhich may be used within the communication system of the presentinvention.

FIG. 3 is a timing diagram showing the timing of control andcommunications channels in a system of the prior art.

FIG. 4 is a timing diagram showing the timing of control andcommunication channels and the timing of receive and transmissionportions of a communication frame which may be used in the presentinvention.

FIG. 5 is a schematic view of the interior of a building showingpossible locations of persons and telephone base stations positionedtherein.

FIG. 6 is a frequency assignment table illustrating the use of carrierfrequencies within an embodiment of the present invention.

FIG. 7 is a pictorial diagram of a base station housing and pluralantennas in one aspect of the present invention.

FIG. 8 is a timing diagram showing a frame which may be used in thecommunications portions between the base stations and the portabletelephones in the present invention.

FIG. 9 is a diagram showing an exemplary chart of the output of acorrelator which may be used in the present invention.

FIG. 10 is a simplified functional block diagram illustrating anotherembodiment of the system of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to FIG. 1, a telephone system using the present inventionmay include a switch 10 which is connected to plural lines or trunks ofa standard telephone network such as the public telephone network in theUnited States. The switch 10 is also connected to plural land-basedtelephones 14 via wires or similar non-propagating media. The switch 10may also be connected through a Base Station Interface Unit ("BSIU") 16to one or more base stations 18 through a base station connector 20.Each base station 18 includes means for wireless communications withplural portable telephones 22, as described further below.

In operation, the switch performs as a standard telephone switching unitsuch as a PBX, a PABX, key-system or the like and functionallyinterconnects telephones 14 with the public telephone network. Such aswitch 10 could be a switching unit such as the unit sold by HarrisCorporation as the "Harris 20/20". Depending on the scope and complexityof the switch, it may have many features available to assist thetelephone user with his calls including memory dialing, call forwarding,voice mail answering and delivery, and the many other features common onmodern telephone switches.

One of the tasks accomplished by the switch is the mapping of in-boundor intercom telephone calls to a desired telephone instrument and theprovision of "outside lines" to system telephones upon request. If anin-bound call is bound for one of the land-based telephones 14, theswitch 10 conventionally rings the desired telephone through a wireconnection and establishes a voice communication path along the same oranother wire connection between the desired telephone 14 and the outsideline on which the call was received.

Calls received by the switch 10 destined for portable telephones 22 arerouted to the BSIU which sends the appropriate control information tothe base station currently controlling the portable telephone 22 throughmethods described below. When a call is to be connected between portabletelephone 22 and another telephone, the switch 10 operates to direct thevoice circuit to the BSIU 16, which in turn directs the voice circuit toa selected base station 18, which in turn sends information regardingthe voice message on the voice circuit to the desired portable telephone22 via a wireless link 24.

As is well known, telephone voice circuits can be either analog ordigital. In one conventional system, the telephone lines arriving fromthe telephone network may be analog lines. Through conventional switchcircuits, the switch 10 may route the analog voice circuits arrivingfrom the telephone network to the land based telephones 14 in eitheranalog or digital modes. If a digital switch is being used, the switch10 may first convert the voice information arriving from the analogtelephone network to a series of digital bits and communicate those bitsby conventional means to the land-based telephones 14. Communicationsarriving from a digital land-based telephone 14 destined for thetelephone network 12 may be transmitted from the telephone 14 to theswitch 10, converted to an equivalent analog signal and impressed on thelines from the telephone network 12.

The present invention is not limited to use with only analog switches oronly digital switches and the inventive concepts disclosed herein mayreadily be implemented in association with either analog or digitalswitches. Nor is the present invention limited to use with a PBX orsimilar "stand-alone" equipment but may be applied to any telephonesystem or sub-system in which it is desired to integrate a wirelesstelephone with other telephones, wireless or landbased. Thus, the switch10 may itself communicate with the "external" telephone network 12through any conventional means, not limited to wired systems, such asvia microwave links. Thus, while the descriptions of a preferredembodiment may describe a particular signal within a switch 10 as beinga digital signal or an analog signal, the invention is not limited tosystems having only such signals, the description being merelyillustrative and not limiting.

Analog telephone calls from the telephone network 12 destined for one ofthe portable telephones 22 may be converted to a digital form within theswitch 10 and are passed to the BSIU 16. At the BSIU, the digitalinformation from the call may be combined with similar digitalinformation from other calls and passed via the communication links 20to the base station 18 then handling the call. In turn, and in a methoddiscussed below, the base station 18 may communicate the digitalinformation from the telephone call to the desired portable telephone22. Note that as the present system is a telephone system, there existsa return path for the telephone call which simply reverses the flowdescribed above to provide the digital form of the voice applied at theportable telephone 22 through the base station 18 and the BSIU 20 to theappropriate line from the telephone network 12. If the telephone networkline is an analog line, the digital form of the return signal will beconverted to the analog form for impression on the telephone networkline 12.

As is well known, even though a voice message may be thought of ascontinuous, the digital form of such a message does not need to be acontinuous stream of digital bits. Rather, if the digital equipment issufficiently fast, clusters of digital bits representing the voicesignal may be transmitted together followed by a period of time beforeanother cluster of bits must again be sent. If the series of clusters ofdigital bits arrives within a certain latency period between clusterswhen the digital signal is converted back to analog, a user listening tothe received analog signal cannot detect the fact that the voice messagewas transmitted in separate portions. The present invention utilizesthis fact to permit multiple portable telephones to simultaneouslytime-share a single frequency or channel between a base station 18 and aportable telephone 22.

With reference now to FIG. 2, in a form of communication known asTime-Division-Multiple-Access, plural portable telephones may share asingle frequency. As depicted in FIG. 2, a particular frequency (orcarrier) may be divided into plural slots, eight (8) in the system ofFIG. 2. With proper coordination between a base station 18 and aportable telephone 22, the portable telephone may expect to receive itsdigital messages in slot 2, the slot repeatedly occurring as one ofeight slots. Thus, the portable telephone 22 receives the digital datain the slot 2 as shown in FIG. 2. Seven slots after the end of the firstslot 2, a second slot 2 arrives, again bearing digital data for thespecified portable telephone 22. Each slot can deliver plural digitalbits of data. So long as the slots repeat with sufficient frequency, theportable telephone 22 will receive the digital form of the voice messageat a sufficient rate that the analog form of the message may bereconstructed accurately by a digital-to-analog conversion at theportable telephone 22. The length of time which may pass beforeconsecutive slots must be received to avoid a loss of data depends uponhow many bits are sent at one time and the frequencies of the soundswithin the voice message in a well known relationship. As notedpreviously, a typical telephone message is two-way and thus, two slotsare needed for a telephone call, one for the passage of the digitalsignal to the portable telephone 22 and the other for the passage of thedigital signal from the portable telephone 22.

Because one telephone call occupies only two slots of the frame, theother slots are available for simultaneous usage by other portabletelephones 22. Generally, the base unit uses a particular protocol toinform the portable telephones 22 which slots are to be used for aparticular telephone call and to ensure that all of the communicatingportable telephones are synchronized (to avoid having two portabletelephone transmitting at exactly the same time which would usuallydestroy the signals being transmitted by both.)

In prior art systems, typically, the first slots of a frame are used forsending voice signals from the base station to the various portabletelephones then being controlled by it. For example, and with referenceto FIG. 3, an 9 slot frame of the prior art may use the first slot (CON)as a control slot (for sending control messages from the base unit tothe stations, etc.), the next four slots for voice signals going fromthe base station to the portable telephone (T1, T2, T3, T4) and the lastfour slots for voice signals going from the portable telephones to thebase station (R1, R2, R3, R4).

In the system of the present invention, and as depicted in FIG. 4, thefirst slot of a frame (CON) may be used as a control slot; however, incontrast to the prior art, the remaining slots are divided into twohalf-slots with the base station transmitting in the first half-slot andthe base station receiving (portable telephone sending) in the secondhalf-slot of each slot.

Compared with the prior art, the use of the present invention isparticularly beneficial in reducing power consumption by a portabletelephone. It is known in the prior art to reduce the power supplied tothe communication portions of a portable telephone when thecommunication portions are not expected to be needed. Thus, for examplethat in the prior art system of FIG. 3, a portable telephone couldreduce the power to its communication portions in all but three slots,i.e., the slots when it is scheduled to transmit, to receive and tocommunicate within the control slot. In one aspect of the presentinvention, the communication portions of the portable telephone arerequired to be fully energized only during one slot (as explained below,the portable telephone of the present invention may not need to monitorthe communication channel (or slot) while it is engaged in a telephonecall. Thus, the present invention reduces the relatively inefficientperiods during which power is being brought up and reduced from thecommunications circuits in addition to reducing the need to power upsuch circuits at all during fewer slots per frame.

As is known in prior art systems, the control channel can be used toestablish timing for the frames of the communications system and forcommunicating control information between the base station and theportable telephones. For example during the periods of the controlchannels, the base station can transmit data informing particularportable telephones which slots and frequencies to use for their voicedata or informing a particular telephone how to react to certain userentries of the telephones keypad. Also during the control slot, thesystem may provide a period of time during which any portable telephoneseeking a voice circuit may transmit its request for such a circuitand/or may transmit other administrative data or functional requests ofthe base station or PBX.

With reference now to FIG. 5, the ability of a system of the presentinvention to provide continuous coverage of an area may be illustratedon an outline of a typical floor of an office space 50. Within theoffice space 50, a number of walls 52 divide the space into individualrooms 54 and corridors 56. Base Stations BS 1 and BS 2 can be placedwithin the office space 50 to provide telephone coverage throughout. Thenumber and location of Base Stations depends upon the amount oftelephone traffic expected and the configuration of the space to becovered. The space may be a portion of a single floor of a building,multiple floors of a building, multiple buildings, and/or external spaceassociated with the buildings. When a user desires to use his portabletelephone within the space covered by the system of the presentinvention, he may enter a set of instructions onto his portabletelephone. By a process discussed below, the portable telephone selectsa Base Station with which to communicate and sends the user's requestfor a voice circuit to the selected Base Station during one of thecontrol slots associated with that Base Station. The Base Stationdetermines which slots and frequencies are available and returns amessage to the portable telephone identifying the slot and frequency tobe used by the portable telephone as a voice channel. Through standardsignalling techniques, the portable telephone may then transmit thesubscriber number of the telephone to be dialed or request some otherfunction of the telephone system.

By way of example and with continued reference to FIG. 5, each of theBase Stations within the system may have associated with it a frequencyto be used for its control channel. When the portable telephone seeks avoice circuit or other function from the telephone system, the portabletelephone may scan those frequencies known to be assigned as controlchannels to determine which control channel signal is received with thehighest signal strength, signal to noise ratio, and/or any other metricof signal quality. The control channel signal with the best qualityhaving been selected, the portable telephone will then initiate contactwith the Base Station sending that signal via the control channel. Withrespect to the office space of FIG. 5, portable telephones PT1 and PT3may be expected to communicate with Base Station 1, if that base stationhas an available channel, because that base station is closer than anyother, otherwise portable telephones PT1 and PT2 may be expected tocommunicate with Base Station 2. Similarly, portable telephone PT4 maybe expected to communicate with (and obtain a voice channel from)communicate with Base Station 2 for the same reasons. The base stationwith which PT3 and PT5 would communicate cannot be readily determinedfrom the outline of the space of FIG. 5. Note that portable telephonePT5 is physically closer to Base Station 2 than to Base Station 1;however, the direct path between the portable telephone PT5 and BaseStation 2 is through a wall. Depending upon a myriad of factors,including the thickness and composition of the wall and the size of thedoor opening to the corridor from the room in which portable telephonePT5 is located, either Base Stations 1 or Base Station 2 (if available)might be where portable telephone PT5 lands.

Note that by the use of the selection scheme of the present invention,the assignment of portable telephones to particular base stations isdynamic and takes into account local variations in the signalscommunicated between the portable telephone and the base station.

In one embodiment of the present invention, incoming and outgoingtelephone calls can be handled with practically the same processes atthe portable telephone, the difference being only that the incomingtelephone calls are preceded by a "Ring" signal. Upon the receipt of the"Ring" signal, the present invention may use the same protocol in theportable telephone for both incoming and outgoing calls.

During a telephone call at a particular portable telephone, the user ofthe portable telephone may be moving about the office. As the usermoves, he will at some time move away from the base station controllingthe call. In the system of the present invention, the portable telephonemonitors the quality of the communications between the portabletelephone and the base station and, upon the degradation of the signalbelow predetermined criteria, reselects its base station and requeststhat the call be moved to a new channel (or slot/frequency combination,explained below) at the base station then having the best communicationswith the portable telephone. For example, and with reference again toFIG. 5, a portable telephone at the location designated PT1 within thespace may be expected to initially use the nearest base station BS1(assuming that BS1 has an available channel). If the user of theportable telephone then moves during the call from PT1 to PT4, thesignal between the portable telephone and its controlling base station(BS1 in this example) may degrade sufficiently to cause the portabletelephone to seek a new communication path. At the position designatedPT4, the portable telephone may be expected to select the nearest basestation, now BS2, and obtain a voice channel from the newly selectedbase station. Note that in the system of the present invention, theportable telephone need not switch base stations merely because the usermoves his location and another base station becomes closer and/or moredirect. Rather, to avoid unnecessary hopping from one channel toanother, the portable telephone of the present invention may continue tostay with its present base station so long as the quality of the voicecommunication remains acceptable.

The portable telephone may evaluate any characteristic of thecommunications channel between the portable telephone and the basestation. It has been found efficient in the present invention to use thevoice quality estimate typically provided by a vocoder in thetransmission circuit as an estimate of signal quality. The use of thisestimate for control of hopping is particularly efficient because mostvocoder circuits already develop the estimate as part of their volumecontrol operation and thus this use avoids the need for another circuitor processing function.

In the standby mode (no on-gong voice communications), the portabletelephone may estimate signal quality based on the quality of thecontrol channel from the base station. If the signal quality degrades toa predetermined criteria, the portable telephone may switch to thecontrol channel of another base station. This technique, sometimescalled soft roaming, may prevent a transient effect which can occur whena call is initiated or received by a portable telephone just before thesignal quality in fact degrades to below the level needed for seeking ahop. In such a situation on a newly established call, the portabletelephone may not have obtained sufficient useful call quality data onthe communications channel to accurately initiate a basestation/frequency-slot switch. By using this technique during stand-bymodes, the system of the present invention attempts to increase theprobability that the portable telephone will use the best base stationnearby.

By remaining in a given voice channel so long as the channel hasacceptable communications, the portable unit typically does not need tomonitor the control channel during a telephone call. Because theassigned slot occurs at a known repetition rate, during a voicecommunication the portable unit may deenergize its communicationcircuits except during the slot in which it is communicating. Upon theend of a slot, the portable unit can deenergize its communicationscircuits and set a timer to reenergize such circuits immediately beforethe next occurrence of the assigned slot. In the time periods betweenthe slots, the portable telephone continues to convert any digital voicedata received from the forward channel (the channel from the basestation to the portable telephone) from digital to analog form forapplication to the telephone's speaker (or earpiece) and to accumulatedigital information corresponding to any audio inputs (or user keypadentries) received during the time period between slots. When the nextslot occurs, the portable telephone receives the next packet of forwardchannel voice data and transmits the next packet of voice data inputfrom the user during the inter-slot periods. So long as the clockingcircuits within the portable telephone remain sufficiently synchronizedto the clocking circuits of the base stations, the portable telephonewill be fully able to jump directly from one slot to the same slot inthe next frame by use of timing circuits and without need to consultwith the control channel.

With reference now to FIG. 6, in a system of the present invention, aslot or channel may be considered to be a particular slot/frequencycombination. In a system having eights slots per frame, there are eightchannels or communication opportunities available per frame, each"opportunity" including a forward communication and a reverse (from theportable telephone to the base unit) communication. Within a slot, theforward and reverse communications are carried out at the same carrierfrequency. Thus, each base station has eight slots under its control.Each of the slots is independent of the other slots within a frame andeach slot may be at any of the channel frequencies used within thesystem. Thus, for example for base station 2 in the system of FIG. 6,slot 1 (generally the control channel, but see below) may be the controlchannel operating a particular frequency, f6. At the time of the exampleof the system of FIG. 6, base station B also had three voicecommunications ongoing, one portable telephone using frequency f13 onslot 3, another using frequency f2 on slot 6 and still another usingfrequency f3 on slot 7. Each of the slots starts a predetermined timeafter the start of the frame. Thus, when a portable telephone isinstructed by the base station to "use slot 6, frequency f2", theportable unit knows that it will receive forward voice data on frequencyf2 the predetermined period of time associated with slot 6 after thebeginning of each frame and that it is to send its reverse data onfrequency f2 during the appropriate time within slot 6.

In the system of the present invention, any of the assigned frequenciesmay be used in any slot at any base station. Accordingly, there are noassignments of frequencies or sets of frequencies to particular basestations This feature of the present invention provides considerableease in installing the system, a replacement base station or a new basestation. In addition, this feature of the present invention providesconsiderable flexibility of the system to respond to highly mobile usersand shifting service demands through the geography of a given system.

In operation, a base station scans the channels assigned to the systemto determine available slot/frequency combinations. By tuning to each ofthe assigned frequencies during its non-used slots, the base station candetermine which slot/frequency combinations are not being used withinits range and can assign one of those available slot/frequencycombinations when a new voice channel is requested by a portabletelephone. Significantly, the available slot/frequency combination datais done by each base station on a local basis. Because of the limitedrange of the base stations and the portable telephones, the fact that aparticular slot/frequency combination is being used within the systemdoes not necessarily mean that it cannot be used elsewhere in the systemFor example, with reference to the system of FIG. 6, base station A canassign one of the portable telephones being controlled by it to use slot5 and frequency f9 and the same slot and frequency can be used in arelatively remote base station area such as in base station N of thesystem of FIG. 6. As long as the base stations and their controlledportable telephones are sufficiently remote, frequency/slot combinationscan be used and reused multiple times throughout the system of thepresent invention, providing substantial capability for a system tohandle many simultaneous telephone calls.

Generally, the system of the present invention may use the monitoring ofthe system during unused slots to determine whether a slot/frequencycombination is available to it. If the base station cannot detect theon-going use of a particular slot/frequency, that slot/frequency may beconsidered "available" despite the fact that a relatively remoteportable telephone and base station are using the same slot/frequencycombination. In such a system, it is possible that even though aparticular base station is sufficiently remote from a particular use ofa slot/frequency combination that it believes the combination to beavailable, the portable telephone being controlled by that base stationmay receive interference from the "remote" use of the sameslot/frequency combination. However, because, as noted previously, theportable telephone is monitoring the quality of the voice signal beingcommunicated during its assigned slot, such interference if it occurredwould cause the portable telephone to initiate a hop to anotherfrequency/slot combination at the same or another base station.

In yet another aspect of the present invention, the capacity of thesystem to carry telephone calls may be improved by the use ofdisappearing control channels at each of the base stations. When all ofthe non-control channels are being used for on-going telephone calls,the base station is free to assign its control channel as a voicechannel As noted earlier, once a telephone call is established on one ofthe slots, the portable telephone with that call has no further need touse the control channel of the base station. If all of the "normal"voice channels are filled and only the control channel remains at a basestation, once the base station assigns that channel as a voice channel,there simply is no need for a control channel at that base station solong as all the channels remain occupied with voice communications. Anyportable telephones desiring a new voice channel within the geographicarea serviced by a base station which has given up its control channelwill automatically not detect the base station as being available andwill select another base station with which to communicate on itscontrol channel.

As mentioned previously, power reduction is an important considerationin the design of the portable telephones. In another aspect of thepresent invention,power is reduced when a portable telephone is in a"stand-by" mode. In this mode, the portable telephone is energized butvoice communications are not occurring, principally while awaiting a newtelephone call or for the user to place his next telephone call. In thestand-by mode, a portable telephone in accordance with one aspect of thepresent invention does not monitor every frame of the communications butrather monitors every Nth frame and upon completion of the monitoring,reducing power to the communications circuits and setting a timer tore-power the communication circuits after the passage of N-1 frames. Theseries of N frames is called a "superframe". In this way, communicationpower consumption is reduced substantially, yet the telephone remainsavailable to "answer" calls and to readily place calls upon userrequest.

In one known problem in the prior art in communications between pluralremote devices and a base station, all or many of the remote devicesattempt to communicate with the base station's control channel atapproximately the same time, colliding with each other's messages andpermitting few or none to successfully communicate with the base unit.Recall that in a system of the present invention, the portable unitsattempt to communicate with the base station by transmitting during aparticular time period during and timed from the beginning of thecontrol channel slot. If more than one portable telephone attempts totransmit simultaneously, neither may be successful. The problemsassociated with collisions on the control channel can be particularlytroublesome immediately after system turn on or reset when all of theportable telephones may attempt to re-establish contact with the basestations. To reduce the collision on the control channel, a portabletelephone of the present invention uses a number assigned to it todetermine in which frame of a superframe to attempt to contact the basestation's control channel. The number assigned to the portable telephonemay be programmed into one of its memories or may be input by way of DIPswitches which can be read by the microprocessor within the telephone orany similar semi-permanent (or permanent) method. In the system of thepresent invention, the base station indicates in the control channel theposition of each frame within the superframe. For example, if asuperframe consists of 32 frames in a particular system, the controlchannel communication would include an indication that the successiveframes numbered from 1 to 32. Portable telephones may divide theassigned number by a modulus number, usually the number of frames in asuperframe, and attempt to access the control channel during the framehaving the number equal to the modulus remainder. For example if aportable telephone has been assigned the number 179 in a system having32 frames per superframe, that portable would attempt to communicatewith a base unit during the 19th frame of the superframe (i.e., 179modulus 32 leaves a remainder of 19). Note that there is no need foreach portable telephone to have a unique number so long as the numbersare assigned in a fashion which spreads the attempted accesses to thecontrol channel over the entirety of the superframe.

In the event that collisions occur on the control channel even with theadvantages of the present invention, a fallback contention scheme can beused such as the scheme used in "slotted aloha" systems in which eachcolliding portable telephone waits a random period of time beforeattempting to access the control channel in the expectation that therandomness of the period will eliminate or reduce further collisions.

It is well known in telecommunications systems that electromagneticsignals transmitted from one device often do not travel directly to areceiver but follow various paths bouncing from one object or surface toanother until received. Multipath is a particularly significant probleminside a building where a signal will bounce off walls, penetratethrough walls and bounce off other surfaces until received. During thebouncing and at the receiving antenna, the various multipath signalscombine and interfere with each other so that the received signal isusually plural copies of the original signal arriving at differentphases with various cancellations within the waveform. To reduce theeffects of multipath, it is known in the prior art to use pluralantennas at the receiver to provide both spatial and frequencydiversity. In many such systems, the receiver contains logic circuitswhich select the antenna having the best signal, accepting the signalfrom that antenna and rejecting the other signals. In other antennadiversity systems in the prior art, the receiver's circuitry attempts tocombine the signals appearing at the diverse antennas. Usually in theprior art, the latter systems have provided better reception but at acost of increased complexity and circuitry.

The problems associated with multipath are often exacerbated in aportable system in which power levels are generally low (to conservepower and permit frequency reuse) and in which the portable telephone isnot fixed in space. It is a frequent occurrence that the user readjuststhe position of the portable telephone during a conversation, each timerealigning the antenna with respect to the base station (generallyinternal to the telephone or fixed externally in orientation to thetelephone). In addition, the user is often moving which will causecoriolis effects in the transmitted signal. Thus, it is particularlyuseful in portable systems to use a diversity of antennas at both thebase station and the portable telephone.

In another aspect of the present invention, the system of the presetinvention uses "duality" to improve signal reception. With reference toFIG. 7, each base station 70 may include plural antennas 72 associatedtherewith. During each slot in which the base station is communicatingwith a portable telephone, the portable telephone first communicateswith the base station. During the preamble portion of the communicationfrom the portable telephone to the base station, the portable telephoneincludes within the preamble a predetermined bit pattern which the baseunit uses to test the reception on each of its antennas (four in thesystem of FIG. 7) The base station then uses the best antenna to receivethe rest of the communication from the portable telephone. During thesame slot, the base station has an opportunity to communicate forwarddata to the portable station and will use the same antenna to transmitsuch data as it just used to receive data from the portable unit.Because the base station transmit portion of the communication to theportable telephone immediately follows the receipt of a communicationfrom the portable telephone and an evaluation of the best antenna to beused, it is unlikely that the portable station has changed its positionsuch that the previously selected antenna is no longer the best one tobe used between the base station and the telephone.

The base station can use the same antenna evaluation and selection foreach of its slots during a frame. Thus, it is entirely possible that thebase station would select a different antenna to be used for eachsuccessive slot.

It has been found by the present inventors that during antennaselection, the antennas may be switched while still measuring theirsignal quality. The delays inherent in the switching chain and thedelays in the measurement of the received RF signal, permit testing tocontinue for some period of time after an antenna has been commanded tobe switched out of the communication. The advantage of such a techniqueis the ability to test more signal data without having to dedicate moreof the preamble to antenna diversity functions.

In another aspect of the present invention, the antenna selection systemmay select the two best antennas for a given slot, using the bestantenna for transmission to the portable telephone later in the slot andevaluating the two best antenna for attenuation setting evaluation. Ifthe signals received at the two best antennas exceed a predeterminedfigure of merit, an antenna can be added to the signal path during thenext frame. On the contrary, if the two best signals are weaker than apredetermined figure of merit, the attenuation may be removed in thenext frame. In this way, the dynamic range of the reception may beenhanced as the attenuation is selectively added or removed based on thestrength of the two signals. The use of the signals from two antennas isnot meant to be limiting to the invention and more signals may be usedif desired, at an increased processing burden. Similarly, the figure ofmerit may be related to the strength of the two signals, individually orcombined in a desired manner, or to any other characteristic of thereceived signals.

With continued reference to FIG. 7, a base station of the presentinvention may be housed in a housing 76 of any suitable configuration,and desirably of a configuration which blends well into typical officedecor. The base station 70 may include plural independently attachedantennas 74 which can be pivotally moved from a position nestled alongthe side of the housing 76 (as seen with respect to the antenna 72a) orperpendicular thereto (as seen with respect to the antenna 72). Theantenna 72 may be attached to the housing through any conventional meanswhich permits rotation about a pivot 74 while maintaining electricalcontact between the antenna and the circuits within the housing 76.Optionally, the antenna may be attached to the housing by otherconventional means which permits individual alignment such as by a balland socket arrangement (not shown) which would permit rotation aboutmore than a single axis. Indeed, physical attachment of the antennas tothe housing is not necessary so long as an electrical path is maintainedbetween the antennas and the base station communication circuits.

In operation, the antennas 72 of the base station 70 may be aligned inany manner which increases the ability of the base station 70 tocommunicate with the portable telephones. For example, in a system ofthe present embodiment, the base station and the portable telephones maybe designed to use vertical polarization. If the base station is mountedon a ceiling, all of the antennas 72 may be swung down to the verticalto provide an orientation which matches the system design. If the basestation is affixed to a vertical wall, the bottom set of antennas may beleft alongside the housing 76 while the two top antennas may be swungout 180 degrees to a vertically-upward position. Similarly, the antennasmay be placed independently in any position relative to the housing 76which increases signal throughput to a portable telephone to adjust foranomalous RF propagation conditions in the geographic area near the basestation 70.

One means for implementing the signalling between a portable telephoneand a base station of the present invention is illustrated in the timingdiagram of FIG. 8, which depicts the functional composition of one-halfa slot of the present invention. The time slot may include plural bitsof information, 240 bits are used in one embodiment of the presentinvention. The first portion of the slot comprises a preamble, which mayinclude sufficient synchronization bits to permit the receiver toacquire the signal before the data is received. Following the preamblemay be a "unique word", i.e., a predetermined set of bits which arealways found at the same location in a slot. By use of a unique word ata known position within the slot, the receiver is able to synchronizeits frame clock to that of the transmitter. Following the unique wordmay occur plural "data" bits. The data bits, depending upon the type ofmessage and the on-going activity at the transmitter, may includedigital voice data, control data, command data, or the like (generallyreferred to herein as "application" data). Finally, some type ofchecksum or CRC may be included to assist the receiver in ascertainingthat it has received the message correctly. Generally, any type of knownvalidation scheme may be used within the communications, includingchecksums, check bits, folding checksums, error correction bits, etc.,realizing that there is a tradeoff between the number of bits dedicatedto message validity versus the data throughput of the system.

During the preamble of the message from the portable telephones to thebase units, the four antenna selection fields are included. Note thatbecause the antenna selection bits occupy a portion of the message whichis conventionally used for recovering symbol timing, symbol recovery bythe base station may be degraded unless other measures are undertaken.One such measure could be to increase the size of the preamble relativeto the data portion of the message or to increase the overall length ofthe message. Both these options entail a reduction in the data bandwidthof the system. In one aspect of the present invention, neither bandwidthreducing measure is taken but a novel method of recovering the symboltiming is used. With reference to FIG. 8, in one aspect of the presentinvention, the symbol timing can be obtained from the output of acorrelator through which the signal is applied. In general, the receivedsignal may be sampled to provide plural samples for each symbol. In oneembodiment of the present invention, each symbol of the input signal issampled ten times at the output of the correlator. The magnitude of thecorrelator output at any one time corresponds to probability that thecenter of the received symbol is at that time. For each ith samplewithin the symbol, the total of the samples is obtained and compared tothe total of all the other samples, with the greatest total indicatingthe sample most likely to correspond with the center of the symbol. Inother words, the receiver may calculate the following equation: ##EQU1##where K equals the number of samples per symbol, and TOTAL equals thetotal number of samples per slot (all the symbols). Note that thisaspect of the present invention uses the entire slot to determine symboltiming and then uses that determination to extract each of the symbols.To implement this aspect of the invention, the incoming message may bestored in dynamic memory while the symbol timing is being extracted;however, the symbol timing determination may begin upon initial receiptof the message and need not await receipt of the entire slot.

Once the symbol timing has been obtained the receiver of the presentinvention may use a "moving window" correlator to find the unique word.There is no need in the present invention for a threshold correlator.Thus, the correlator examines finite portions of the incoming signalhaving a length of an entire slot to determine where the unique word ismost likely to be. Once the correlator has determined the most likelylocation, the processor of the receiver may make a bit for bitinspection to the recovered slot to determine whether each bit of theassumed unique word is present in the incoming message. If the uniqueword is found, the receiver may then set up all of its framing aroundthe unique word, as it is predetermined where the unique word appearswithin a frame.

Once the unique word has been successfully acquired, the unique word insubsequent frames may be used to measure the drift between thetransmitter and receiver and to adjust the receiver for that drift. Theacquisition of the unique word in subsequent frames does not need toexamine the entire frame, however, as a small window around the expectedtime for the unique word to appear will usually suffice and reduce theprocessing burden on the receiver.

To assist the base stations in maintaining synchronization with eachother so that they can correctly examine slot/frequency combinations, asystem of the present invention may include a common clock pulse whichis sent periodically to all the base stations. The clock pulse may bederived from any relatively stable source such as from typical switchused in association with the base stations. For example, in a PulseCoded Modulation ("PCM") system such as is often used in digitaltelephony applications, one of the PCM slots between the base stationand the switch may be periodically programmed to carry a synchronization(or reset) pulse which is received by all base stations and used tosynchronize its clock and the communication frames derived therefrom.

The use of a single stable clock helps assure data alignment and reducesthe hardware costs as the base stations may not need an expensiveoscillator if provided with an accurate clock from the switch. Stillfurther, the portable telephones may use a relatively inexpensive, andrelatively less accurate, on-board reference clock in the presentinvention by frequency correcting the reference clock using the receivedRF carrier from the base station (which, in turn, is locked onto theaccurate clock, such as the PCM clock, from the switch).

In another aspect of the present invention, a system may include aprotocol in which the remote unit, upon the initiation of the user, mayinvoke a bit-error-rate ("BER") test of the communications between theremote unit and a base station. This BER test may include thetransmission by the remote unit, the base station or both ofpseudo-random sequences. Both the base unit nd the remote may use thereceipt of such sequences to determine the BER and (in the case of thebase station) may report the BER to the remote unit for display to theuser. The displayed BER information is very useful to system installersto determine and/or verify system coverage and performance. This aspectof the present invention is not limited to BER measurements and a systemof the present invention may send signals and take signal measurementsof any predetermined characteristic of the RF signal, such as signalstrength.

In still another aspect of the present invention, messages between thebase stations and the portable telephones may include a systemidentifier which has been programmed into the portable telephones. Thesystem identifier may then be placed in the messages, for example in thepreamble. Portable telephones and base stations may then examine thesystem identifier and if it does not match the system identifierassociated with the receiver may ignore the message entirely. Thus, twototally separate systems may exist side-by-side to each other coveringportions of the same geographic areas. Similarly, the portabletelephones may be programmed to operate on more than one installedsystem and would thereby be free to roam from one system to another.Depending upon traffic and security considerations, persons may beprovided with portable telephones which varied in their abilities to beaccessed from various geographic areas by changing the systemidentifications which are authorized for particular portable telephones.

Similarly, the portable telephones of the present invention may be usedon multiple wireless systems without requiring the user to initiate anyaction other than to energize his remote unit. (The administrator ofeach system providing access to the portable telephone must configurethe system to accept the portable telephone.) This feature isparticularly useful in installations in which different systems operatein different locations within the same (or nearby) business campus. Insuch an installation, the user may travel from one area of the locationto another, in which a wholly different wireless system has beeninstalled, and still have full use of his portable telephone, includingthe personal features associated with that user. On the other hand, iftwo adjacent systems do not wish to authorize the users of one system touse the other, the wireless system of the present inventionautomatically ignores unauthorized portable telephones and the portabletelephones do not attempt to use the base stations and communicationschannels being used by the "foreign" system.

The system of the present invention may also be used simultaneously as apager system and a telephone system. In this aspect, the portabletelephones may receive an alphanumeric message for their displays andthe ringer can be energized to alert the user of the message. When usedas a pager, the mobile station does not complete a telephone call to theuser at the portable telephone but merely uses the existingcommunication system to forward the pager message. The remote units usedin a combination pager/telephone wireless system may be the same for allusers, or optionally, some of the units used exclusively for paging maybe reduced-cost units lacking the voice communications capability.

With reference again to FIG. 1, installation of the system of thepresent invention may be made relatively easy by its architecture. Forexample, the switch may be a digital switch having associated therewitha BSIU 16 capable of communicating with the base stations 18 through acommunication means 20. The communication means 20 may be as simple astwo sets of twisted wires, two of the wires being used for transmissionof data to the base station 16 and two of the wires being used fortransmission of data to the BSIU 16. The transmission of data on thefour wires can be made using a protocol known as E-1 to handle theplural digital messages which may be simultaneously occurring on eachpair of wires. In addition, the four wires could also carry power andground references to the base stations so that integration of the basestation into a buildings electrical wiring is not necessary.

In operation, a base station 18 can be mounted on any convenientsurface, such as a wall or ceiling, and four wires carried between thebase station 18 and the BSIU 16. By use of a generalized power-upProgrammable Read Only Memory ("PROM") associated with the base station18, the base station 18 may boot itself using the PROM upon theapplication of power from the communication means 20 or from theavailable electrical wiring. Once the base station 18 has booted, it maythen send a message to the BSIU requesting a download of the systemunique data and configuration. Because the base station 18 does not havefrequencies assigned and operates primarily on an evaluation of signalstrength rather than telephone location, no extensive programming of thebase station is generally necessary. This lack of programming necessitymakes installation of the system easier and provides for automaticresumption of service after a power failure or surge.

In still another embodiment of the present invention as depicted in FIG.10 (using common reference numerals as used in FIG. 1 for commonelements), a switch 10 used in the present invention need not be adigital switch as described above but may be a conventional analogswitch which interconnects the public telephone network 12 to the systemIn such a system, a Base Station Interface Apparatus ("BSIA") 90 may beused to interface between the digital wireless system and a conventionalanalog switch 10. The BSIA provides the wireless system (the basestations 18 and portable telephones 22) with all of the functions of theBSIU described with respect to FIG. 1. Additionally, the BSIA of thealternative system of FIG. 10, converts all of the digitalcommunications to and from the portable telephones to conventionalanalog communications and may convert function requests from theportable telephones 22 into standard switch (e.q., PBX) protocols andsignalling. Alternatively, the BSIA 90 may cause some of the functionsto be implemented directly by the portable telephones by appropriatelyprogramming the portable telephones 22 (as explained below) to takespecific, PBX-related actions in response to specified user inputs.

With continued reference to FIG. 10, the BSIA may be configured andcontrolled through a standard personal computer 91 -or similar computingdevice which may be removably attached to the BSIA 90 when desired. Thepersonal computer could be -.connected, for example, when the systemadministrator wanted to make configuration changes, initiate tests orgather data regarding the system's performance. Software within thepersonal computer 91 would typically include software for: SystemConfiguration, Uploading, Analysis of Performance Statistics and Alarms,and Downloading of New Programs to the BSIA and base stations. New orupdated programming for the base stations may readily and convenientlybe implemented by downloading such programs into the BSIA forcommunication to the base stations (or through the BSIU in systems suchas in FIG. 1). Thus, reprogramming of new and improved features does notrequire removal or access to the base stations, resulting in significantsavings for personnel in maintaining installed systems.

The BSIA (or the BSIU) may also include the capability for the portabletelephone user to execute macro commands by simple, brief keyingactions. In the macro capability, the BSIA may provide signals necessaryfor the protocol to the switch to be readily implemented by a singlekeystroke on the portable telephone. For example, if a certain functionin the switch required the successive transmission of the DTMF signalsfor the numbers 1, 2 and 3, a single function key on the portabletelephone could be used to signal the BSIA that the exemplary functionwas desired. The BSIA, in turn, would cause the DTMF signals for thenumbers 1, 2 and 3 to be successively transmitted to the switch. Theswitch receives the DTMF signals, understands that the requestedfunction is desired and performs the actions conventionally necessary toperform the requested function. A set of macros may be programmed intothe BSIA, some of which are portable telephone dependent, so that thesame key on different telephones may cause different signals to be sentto the switch. Still further, differing sets of macros may be developedfor different types of switches to accommodate the differences in theswitch protocols and the appropriate set downloaded into the BSIA duringinstallation.

In another aspect of the present invention, the portable telephone mayinclude a display which can be programmed from the switch (through thebase station) to customize features for the various telephones within asystem. After the system has been installed and energized, message maybe sent from the switch to the telephone to provide certain featureswithin the telephone and may control the appearance of the display onthe telephone. For example, the display of the telephone can be made toresemble the keypad and switches of a conventional land-based telephone.By specifying the functions available to the telephone user, the switchmay make the switches of one telephone operate wholly differentfunctions than the switches at other telephones and may cause thetelephone displays to show different images indicative of the functionsprogrammed. For example, in a telephone display which had five virtualswitches thereon, the top switch on one telephone could be used for a"hold" feature and on a different telephone could be used for a"transfer" feature. Accordingly, the display on the first telephonecould be commanded to display "hold" within or near the image of theswitch on the telephone's display while the second telephone could becommanded to display "transfer" at the same location on the display.Changes to the display and features of the various portable telephonesused in the system can be readily made centrally using the switch andthe command structure of the present invention. In addition, if theportable telephone user is required to provide a user-related code, eachtelephone could change its appearance and functions responsively to theuser code and the same telephone would have different features dependingupon the security level or service level of the user.

The display may use both the familiar alphanumeric messages of presentcellular telephones and icons, i.e., graphical images which suggest andrepresent certain states and actions such as "e-mail waiting", batterystrength, speaker volume level, line status, etc.

An embodiment of the present invention may also be configured to utilizeone or more user interface characteristics described below to make theremote units easier to use. For example, an embodiment of the presentinvention may include a user protocol in which the user may either dialthe digits of telephone number to be dialed and then press an action key(such as a "CALL" key) or may first press the action key followed by thedigits of the telephone number to be dialed. The first method resemblesthe user protocol currently common in cellular telephone systems and thesecond method resembles the protocol commonly used in present cordlesstelephones In the present invention, the user may be provided with theoption of using either protocol.

In another useful protocol which may be used in the present invention,the user may press an action key twice to redial the last telephonenumber dialed. For example, the user may press a "CALL" key twice, thefirst time obtaining dial tone and the second time redialing thepreviously dialed number. The use of a single key assists the user bynot requiring search on the keypad for different keys to indicate Calland Redial functions.

In still another useful protocol, incoming telephone calls may beredirected on an ad hoc basis by the user of a remote unit. In thisprotocol, upon the receipt of telephone call at the remote unit, theremote unit may indicate the incoming call by a standard method such asby ringing or vibrating. Optionally, the remote unit may display thetelephone number or extension of the calling party. The user may respondto the incoming call by either: pressing a "Call" key to answer thecall; pressing a "Clear" key to mute the ringing and leave the callunanswered; or, pressing an "End" key to have the call automaticallyredirected to another unit such as another telephone, a messagerecording system, or a voice mail system.

In yet another protocol feature which may be used in the presentinvention, the keyboard of a remote unit may be electronically locked sothat only a limited number of keys my be used. For example, a user mayinput a predetermined key sequence to lock the telephone from all use(except the unlocking functions) or to lock the telephone to arestricted state. In the restricted state, the user may be permitted touse three keys, an "Unlock" key which restores full functions, a "Power"key to turn the remote unit off, and a "Call" key to answer incomingtelephone calls. With the remote unit in the restricted state, theremote unit may be freely carried about without concerns for inadvertentor unauthorized depressions of the keys.

While preferred embodiments of the present invention have beendescribed, it is to be understood that the embodiments described areillustrative only and the scope of the invention is to be defined solelyby the appended claims when accorded a full range of equivalence, manyvariations and modifications naturally occurring to those of skill inthe art from a perusal hereof.

What is claimed is:
 1. A communications system comprising:a telephonicswitch for interconnecting telephone circuits; plural portabletelephones for wirelessly communicating voice signals to plural basestations, said base stations transforming said voice signals receivedfrom said portable telephones to second voice signals and providingcontrol signals and display information to said portable telephones; aninterface device for communicating said second voice signals betweensaid switch and said base stations; plural paging units for wirelesslyreceiving said control signals and operating a user notification devicelocated thereon upon receipt of control signals indicating paging fromone of said base stations; wherein said paging units use the sameprotocol for communicating with said base units as is used by saidportable telephones, whereby a single set of base stations andinterconnecting wiring may simultaneously operate both a telephonesystem and a paging system.
 2. The communication system of claim 1wherein said portable telephones may be used as paging units.
 3. Thecommunication system of claim 2 wherein said portable telephones displayat least some of said display information while being used as a pagingunit.
 4. The communication system of claim 3 wherein said protocolcomprises dividing the communication medium into consecutive frames eachof which is further divided into an equal number of slots, one of saidslots functioning as a control slot until all other slots within theframes are being utilized for communicating application data andthereafter functions as a slot for communicating application data solong as all of the other slots within the frame continue to be utilizedfor communicating application data.
 5. The communication system of claim4 wherein said protocol uses a time-division-multiple access system. 6.The communication system of claim 4 wherein said slot is divided intotwo portions, one portion reserved for communications from saidtelephone to said base station and the other portion being reserved forcommunications from said base station to said telephone.